Belinda Smith
Belinda Smith is a science and technology journalist in Melbourne, Australia.
Around 4.5 billion years ago, an object slammed into Earth vaporising most of the planet into a scorching cloud from which the moon was born.
Geochemists in the US – Kun Wang from Washington University in St Louis and Stein Jacobsen at Harvard – examined minuscule amounts of potassium in moon and Earth rocks and found minute differences – possible only if their raw materials were thoroughly mixed in a superheated fog before they coalesced.
The work, published in Nature, pokes a hole in the theory that the moon was born from a low-impact collision.
The giant impact hypothesis has, for decades, been the frontrunner in how our biggest satellite came to be.
It centres on a glancing blow from a Mars-sized object, which smashed a bit off the Earth but was completely obliterated in the process. Dust and rubble formed a disc around what was left of the Earth, and this clumped together to become the moon.
In the early 2000s, though, geochemists analysed rocks brought back by Apollo astronauts in the 1970s and found isotope ratios of zinc and chlorine, for instance, were identical to those found on Earth. (Isotopes are elements containing different numbers of neutrons in their nucleus.)
The likelihood that the impactor was chemically identical to Earth is incredibly tiny. Chemical make-up is a fingerprint of sorts for objects in the solar system.
This cast doubt on the giant impact hypothesis as it stood at the time. So a couple of modified theories arose.
One involved a low-impact crash, which saw part of Earth melt tossed into a disc of magma. The moon started to crystallise beyond the Roche limit – the distance from Earth where its gravitational field is strong enough to keep pieces from coalescing – and the whole thing was shrouded in a silicate vapour which ferried isotopes between Earth and the developing satellite.
The problem was the speed of the mixing via the silicate atmosphere – or lack thereof, Wang says. There simply wouldn’t be enough time for the material to exchange bits and pieces with the atmosphere before it started to fall back onto Earth.
Another hypothesis was more violent.
The smash was powerful enough to liquidate the baby Earth’s mantle and its impactor into a “supercritical fluid” atmosphere, which expanded into a flattened sphere more than 500 times the volume of Earth today.
This dense melt of hot rock – a cross between a gas and a liquid – mixed quickly and efficiently. And as it cooled, little “moonlets” formed – again, beyond the Roche limit – which eventually clumped to become the moon.
To test these theories, Wang and Jacobsen looked at potassium isotopes in terrestrial and lunar rocks from Apollo 11, 12, 14 and 16 missions – what they call a “palaeo-barometer” for moon-forming conditions.
Of potassium’s three stable elements, only two – potassium-41 and potassium-39 – are plentiful enough to be measured.
The ratio of these isotopes could be the key, they realised. If moon rocks contained more of the heavier potassium-41 isotope compared to the lighter potassium-39, this would point to the violent moon birth story, because heavier atoms would preferentially “fall out” of the cloud and clump together more readily than light ones.
In April, the pair unveiled a technique to detect these negligible scraps of potassium to a precision of 0.05 parts per million – scientists hadn’t had instruments sensitive enough to detect them until then.
And sure enough, the lunar rocks were enriched with heavy potassium by about 0.4 parts per thousand – “the first hard evidence that the impact really did (largely) vaporise Earth”, Wang says.
